Anodic oxidation of ferrophosphorus



Patented Oct. 18 1938 UNITED STATES ANODIC OXIDATION OF FERRO- PHOSPHORUS Lewis H. D. Fraser, Sheffield, Ala. No Drawing. Original application October, 15,

1936, Serial No. 105,810. Divided and this application January 7, 1938, Serial No. 183,794

4 Claims.

(Granted under the act of March 3, 1883, as I amended April 30, 1928; 370 0. G. 757) The invention herein described may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to a process for the anodic oxidation of term-phosphorus and cerifzain new compositions of matter resulting thererom.

This application is a division of my original App. Ser. No. 105,810, filed October 15, 1936.

One of the objects of this invention is to provide a process for the conversion of ferro-phosphorus, a by-product from the production of elemental phosphorus in a blast furnace or an electrical reduction furnace, into useful phosphates. Another object of this invention is to provide a method for the conversion of ferro-phosphorus into phosphates which is more economical than those heretofore used. Other objects of this invention include the production of finely divided iron oxide, the particle size of which is of colloidal dimensions and which may be used as a paint pigment or a rouge, and the production of finely divided basic iron phosphate, the particle size of which is of colloidal dimensions and which may be used as a paint pigment or a fertilizer by the anodic oxidiation of term-phosphorus in aqueous electrolytes.

Four iron phosphides, FesP, FezP, FeP, and FePz, which contain 15.6, 21.7, 35.7, and 52.6% phosphorus respectively, are known. Commercial iron phosphide, commonly called ferro-phosphorus, usually contains 14.5 to 28.5% phosphorus. 'lihe commercial material does not, therefore, contain an amount of phosphorus corresponding exactly to any one iron phosphide. Ferro-phosphorus containing 15.6 to 21.7% phosphorus is usually considered as being a mixture of FeaP and FezP, while material containing more than 21.7% is considered a mixture of FezP and another phosphide richer in phosphorus. Although this description is not limited to ferrophosphorus of any one composition, its iron phosphide content is discussed in an illustrative manner as though it was the iron phosphide, FeP.

I have discovered a process for converting the phosphorus contained in ferro-phosphorus into phosphates by the anodic oxidation of the ferrophosphorus in an aqueous solution of an electrolyte containing an inorganic compound such as an alkali metal hydroxide, an alkali metal salt of an inorganic acid, an inorganic acid, or a mixture of such a salt with an inorganic acid or an alkali metal hydroxide. The use of an alkali metal hydroxide in the electrolyte results use of an alkali metal salt or inorganic acid in the electrolyte results in the formation ofa basic iron phosphate, and the use of inorganic acid in an electrolyte results in the formation of an acid iron phosphate. When mixtures of salts and acids or salts and hydroxides are used as electrolytes the products resemble those'obtained with both of the pure ingredients used as electrolytes individually.

In the several examples which follow, samples of term-phosphorus Were'made anodes in cells containing aqueous solutions of a base, a salt or an acid employing chemically inert cathodes. Electrolysis was carried onin these electrolytes under known conditions of direct current, anode current density, temperature, cell voltage and power input until an appreciable quantity of ferro-v phosphorus had been consumed, at which time the electrolysis was terminated, The electrodes were then removed from the electrolytes and the in the formation of an alkali metal phosphate, the

losses in weight of ferro-phosphorus were determined after cleaning. Any insoluble products present in the electrolytes after electrolysis were filtered out and washed substantially free from the electrolytes with a small amount of water. Wash water used was combined with the filtrate and the total volume measured. The insoluble products were dried, weighed and analyzed for the chemical-constituents of interest (principally 1 iron, total phosphorus, water soluble phosphate and phosphate insoluble in neutral ammonium citrate solution) The filtrates were analyzed for their phosphorus pentoxide contents, reported as grams of P205 per liter of solution, and for any other ingredients of interest. The current and energy efficiencies obtained for the oxidation of the phosphorus and ferro-phosphorus were calculated for each experiment from the ampere hours and kilowatt hours employed, and the amounts of P205 recovered as water soluble phosphate and neutral ammonium citrate soluble phosphate.

One example of the operation of my process is given for the anodic oxidation of ferro-phosphorus in a basic electrolyte. A direct current of approximately 10 amperes was passed for 98.5 hours at an anodic current density of approximately 15 amperes per square foot through ferrophosphorus as an anode and graphite as a cathode in 5 liters of potassium hydroxide solution containing 280.5 grams of KOH per liter. At the termination of the electrolysis, the sludge was filtered, washed and dried. A filtrate, 8105 liters, was found to contain phosphate equivalent to 270 consumption of 3.45 kilowatt hours per pound of total P205 produced. The dried. sludge contained 73.3% Fe'zOa and 17.7% P205, the latter being water insoluble and insoluble in neutral ammonium citrate solution.

Another example for the operation of my process is given for the anodic-oxidation of ferrophosphorus in a saline electrolyte. A direct current of approximately 5 amperes was passed for 137 hours at an anodic current density of approximately 10 amperes per square foot through ferroph'osphorus as an anode and lead as a cathode in 5 liters of a potassium chloride solution containing 372.75 grams of KCl per liter. At the termination of the electrolysis, the sludge was filtered, washed and dried. A filtrate, 4.8 liters, was found to obtain phosphate equivalent to 2.9 grams P205 per liter. The dried sludge contained total phosphate equivalent to 19.03% P205 and 44.5% Fe2Oa. Although the phosphate 'contained in the dried sludge was insoluble in water, 70.5% of the total P205 or 13.4% on the basis of the dry sludge, was nevertheless, citrate soluble and, therefore, available from a plant nutrient standpoint. The amounts of phosphorus converted into available phosphate by the use of 582 ampere hours, as indicated by the increase in weight of the cathode in a copper coulometer which was in series with the cell during the electrolysis, (0.190 gram of P205 per ampere hour) corresponded to a current efficiency of 57.5%. An average cell voltage of 1.9 volts was recorded, corresponding to a total power input of 1.105 kilowatt hours and a power consumption of 4.54 kilowatt hours per pound of total available P205 produced.

Still another example of the, operation of my process is given for the anodic oxidation of ferrophorus in an acid electrolyte. A direct current of 5 amperes was passed for 128 hours at an anodic current density of approximately 10 amperes per square foot through a ferro-phosphorus anode and asheet lead cathode in 5.0 liters of phosphoric acid, "containing 188.1 grams of P205 and 0.34 gram of F8203 per liter. At the termination of the electrolysis the solution resulting, 5.675 liters was found to containphosphate equivalent to 177.2 grams of P205 and 27.6 grams of FezOa per liter. The difference between the final and initial P205 contents of the electrolyte of 55 grams P205 obtained by the use of 582 ampere hours, as indicated by the increase in weight of the cathode in a copper coulometer in series with the cell during the electrolysis, (0.094 gram of P205 per ampere hour) corresponded to a'c'urrent efliciency of 28.4% computed on the basis of the oxidation of phosphorus from a negative valence of 3 to a positive valence of 5. An average cell voltage of 1.7 volts was recorded; corresponding to a total power input of 0.99 kilowatt hour and a power consumption of 8.15 kilowatt hours per pound of total P205 produced in solution.

It is evident that there are numerous factors which will influence conditions for the most satisfactory operation of my invention, the actual limits of which cannot be established except by a amazes detailed study of each set of raw materials and the intermediate and finished products involved.

Ferro-phosphorus is cast in flat slabs suitable for use as electrodes in electrolytic cells. These slabs may be cast after the separation of the ferro-phosphorus' from the furnace slag either at the time of tappingthe ferro-phosphorus from a phosphate reduction furnace or subsequent there to. The ferro-phosphorus slabs are made the anodes of a plurality of cells arranged in series electrically and, in order to obtain the maximum anode area, these slabs are mounted centrally between their respective pairs of cathodes in each cell.

The cathodes are constructed of to the conditions of operation in the electrolyte used. I have found lead and carbon to be generally suitable for this purpose but cathodes made of other material, such as steel and copper for use in alkaline electrolytes, are also suitable. The nature of the cathode will vary depending upon the nature and composition of the electrolyte used.

The electrolysis is carried on continuously with ment of anodes as these are used up or for the removal of electrolyte. The electrolyte can be introduced discontinuously but it is preferable to permit a. continuous admission of the electrolyte and a continuous withdrawal of some of the phosphorus is dependent upon the nature of the original electrolyte used and the type of product desired.

JWhen an aqueous solution of a strong base, such as sodium hydroxide or potassium hydroxide, is employed as the electrolyte, the products obtained from the anodic oxidation of ferro-phosphorus are iron oxide infinely divided form and the tertiary phosphate of the metal of the base originally used as the electrolyte. When the electrolysis has progressed sufiiciently to produce either a desired concentration of phosphate ions in solution or a sludge of desired density, the electrolyte is pumped to a filter to remove the iron oxide sludge and the tertiary. alkali metal phosphate is recovered by evaporation, cooling, crystallization, and filtration from .the mother liquor, which is returned to the electrolytic cells with additional base and water. With proper filtration and washing methods employed, the salt is found to be free from heavy metals and the iron oxide product will have only traces of the alkali metal phosphate present. The solution containing the alkali metal phosphate may be regeneratedafter the iron oxide sludge has been removed by the addition of an alkalineearth metal base, the phosphate of which is insoluble in alkaline solutions. This reaction need not be carried to completion as a small amount of phosphate ion present in the electrolyte returned to the cell has no deleterious effect. By the use of lime a fertilizer product could be produced very cheaply with the purchase of only a sufliciently strong alkali to make up for the small losses inherent in the process.

When the original electrolyte consists of an aqueous solution of a strong base, the iron oxide sludge produced is found to be nearly free of phosphorus at high current densities while at low current densities more unoxidized ferro-phosphorus a conductor of -electricity which is essentially chemically inert the shunting of individual cells for the replaceis found in the iron oxide sludge. The phosphorus oxidation current and energy efficiencies are considerably higher at low current densities (5 to 50 amperes per square foot) than at very high current densities (above 100 amperes per square foot).

When an aqueous solution of a salt of a strong base and a strong acid (such as sodium chloride or potassium chloride) is employed, the electrolyte is found to be changed but slightly in strength on prolonged electrolysis, the major change being the oxidation of the ferro-phosphorus to a mixture of iron hydroxide or. oxide and iron phosphate. When a chloride is'used as the electrolyte the filtrate obtained after the separation of the basic iron phosphate is returned to the cells with the addition of the amount of salt necessary to restore it to its original concentration. The chlorine evolved during electrolysis may be com'- binedgwith a part of the hydrogen produced at the cathode and may be returned to the system as hydrochloric acid.

When an aqueous solution of a strong inorganic acid is used, the product obtained is an acid iron phosphate. With a sufliciently high acidity maintained in the electrolyte the oxidation products are all found in solution in the form of iron ions and phosphate ions, otherwise on continuing the electrolysis, the oxidation products will appear as a water insoluble acid iron phosphate. The use of an aqueous solution of phosphoric acid as the electrolyte results in the final production of water insoluble irong phosphate containing essentially all the phosphorus added as phosphoric acid and as ferro-phosphorus. The use of electrolytes containing sodium hydroxide, potassium hydroxide, sodium chloride",

potassium chloride, and phosphoric acid have been discussed in considerable detail. This invention is not, however, limited to the use of these particular electrolytes since solutions of other salts, such as the nitrate, phosphate, or sulfate, and other acids, such as hydrochloric, nitric, and

sulfuric, may also be used.. Furthermore, the.

glomerate on drying. This agglomerate may be' readily disintegrated so that the resulting powder improved characteristics to the resulting prodmay be used for various purposes, such as pigment for use in paint, rouge or rubber, where the colloidal nature of the material imparts distinctly ucts.

Certainterms used throughout the description and claims are understood to have the following meaning: anode current density is the constant current employed divided by the total immersed surface of the anode and is expressed in amperes per square foot of anode area; current efiiciency is arbitrarily calculated as the number of grams of phosphorus oxidized to water soluble and neutral ammonium citrate solublephosphate divided by the number of grams of phosphorus which would be theoretically oxidized from a negative valence of 3 to a positive valenceof 5 by the use of the number of ampere hours employed, neglecting the ampere hours consumed in' oxidizing the iron content of the ferro-phosphorus; 6. energy efliciency is the amount of phosphorus which is oxidized to water soluble and neutral ammonium citrate soluble phosphate per unit of electrical energy consumed.

It willbe seen, therefore, that this invention actually may be carried out by the modification of certain details without departing from its spirit or scope.

I claim:

1. Process of making acid iron phosphate from ferro-phosphorus, which comprises, making ferrophosphorus'an anode in an electrolyte consisting of an aqueous solution of phosphoric acid, containg 5 to H3PO4; making a conductor, chemically inert as a cathode, a'cathode; and passing a direct. current, atan anode current density of 5 to 50 amperes per square foot, "between the anode and the cathode to produce a solution of the acid iron phosphate,

2. Process of making acid iron phosphate fro ferro-phosphorus which comprises, making a ferro-phosphorus an anode in an electrolyte consisting of an aqueous solution of sulfuric acid,

containing 5 to 75% H2804; making a conductor,

chemically inert as a cathode, a cathode; and

passing a direct current, at an anode current density of 5 to 50 amperes per square foot, between the anode and the cathode to produce a solution of the acid iron phosphate.

3. Process of making acid iron phosphate from ferro-phosphorus, which comprises, making ferro phosphorus an anode in an electrolyte consisting of an aqueous solution of hydrogen chloride, containing 5 to 35% E01; making a conductor, chemically inert as a cathode, a cathode; and passing a direct current, at an anode current densityof 5 to 50 amperes per square foot, between the anode and the cathode to produce a solution of the acid iron phosphate. I

4. Process of making acid iron phosphate from term-phosphorus, which comprises/making ferrophosphorus an anode in an electrolyte consisting of an aqueous solution of astrong mineral acid; making 'a conductor, chemicallyinert as a cathode, a cathode; and passing a direct current, at an anode current density of not morethan amperes per square foot, between the anode and the cathode'to produce a solution ,of the acid from phosphate.

rlawis H. D. mama 

